If you have access to a saw, trim off the corners and ends. Whatever you can trim off means less to sand off, which takes more effort.
If you have access to an electric drill, you could use a sanding drum to make the sanding go faster. A sanding drum is a small cylinder with a drill bit sticking out one end. They are usually made of rubber or plastic. Some are designed so that you slide a sandpaper sleeve on the rubber cylinder. Others have a small clamp, and you wrap a strip of sandpaper around it.
Put the sanding drum in your electric drill. Find a way to clamp the wood so it won't move as you work on it. Wear safety glasses and a dust mask. Turn on the drill and use the spinning sandpaper to shape and smooth your curve. With the softwood that you are using, it will go quickly.
At the end, you will want a sanding block (just a piece of wood that fits your hand) and some sandpaper to finish it with a little hand sanding. Use increasingly finer grits (start with 80, then 120, then 150, then maybe 180 or 220).